Proper calibration of a radionuclide emission tomography camera is essential for accurate imaging of an object. Presently, calibration of tomography cameras is performed at the manufacturing site before the camera is shipped. Complex, bulky equipment is required to accomplish the calibration.
Calibration is accomplished at the manufacturing site by removing the standard collimator which is positioned between a radionuclide source and a detection device such as a scintillation crystal. Typically, a first calibration grid is installed having a fixed, uniform matrix of holes positioned in alignment with the centerline of photomultipliers of the camera. Scintillations emitted from the detection device are measured by a number of photomultipliers which are coupled to the scintillation crystal by a light pipe. The outputs of the photomultipliers are then initialized to match the energy levels of detected photopeaks. Alternatively, a single point source of radionuclide emissions is successively positioned in front of each photomultiplier.
A second grid having a fixed, uniform matrix of holes is then utilized for linearity corrections, that is, corrections for spatial distortions in the position estimation of a scintillation event. It is desirable for the second fixed grid to have holes more closely spaced than those of the first grid. However, the distances between the holes must be sufficiently great to enable resolution of the different hole locations: the camera must be able to match an estimated hole position with the actual hole position. If the holes are too closely spaced, a detected event cannot be correlated with the actual location of the event. Correction tables are developed to compensate for nonlinearities in position estimation.
In summary, a first fixed grid is typically used to initialize the photomultipliers, after which position analysis is performed using a second fixed grid to derive correction tables for correcting position estimates made by the camera. The spacing of holes in the fixed grids are limited, however, by the need to accurately resolve the position of an event. In other words, if the holes are spaced too closely together, and an event is detected among several known hole locations, it is difficult to determine which known hole location to match with the detected event.
After the camera is shipped and installed, however, additional calibration may be required. Distortion of the light collection optics or changes in photomultiplier operation can arise during use of the camera. While some cameras can be serviced on site, the servicing cannot be performed by the regular operators of the system. Instead, additional calibration equipment and skilled calibration personnel are required to perform the on-site calibration. Such a procedure is expensive and time-consuming, and does not encourage the camera user to perform routine calibration.
Errors in accuracy can arise from several causes. The gain of the photomultipliers can change over time which adversely affects energy resolution. Further, the junction between a photomultiplier and the light pipe can deteriorate such as by the shifting of gel; the gel normally enhances coupling of the photomultiplier with the light pipe. The uncoupling of the photomultiplier changes the collection characteristics for that photomultiplier which in turn creates nonlinearities in position estimation. Also, the photocathode in the photomultiplier is continually eroding and can erode sufficiently to adversely affect photomultiplier performance. Additional distortions can arise from a fissure in the scintillation crystal or by changes in the reflection of light within the system. Without routine calibration, however, such distortions can pass unnoticed during use of the camera even though the accuracy of the camera can be seriously diminished.